FIELD OF THE INVENTION
The present invention relates to communication systems, and more particularly, systems intended to assure secure communications through radio links.
A continuing problem with telephone systems that employ a radio link, such as cellular systems or radiotelephone, is the danger of unauthorized interceptions of telephone conversations. Because the radio links that are used by the telephones can be received on virtually any receiver that is tuned to the radio frequencies which are utilized for such transmissions, there can be no realistic expectation of privacy.
Conversations conducted over regular telephone land lines, including those with microwave links, are usually clothed with a reasonable expectation of privacy and therefore such conversations frequently include privileged information without risk of the loss of such privilege. Unfortunately, this expectation is not a reasonable one where radio links are involved. For example, a telephone conversation between attorney and client, doctor and patient, priest and penitent or husband and wife, which would normally be privileged, may lose the privileged status if one or both of the parties uses a cellular telephone or other telephone with a radio link.
It has long been known that telephone conversations containing extremely sensitive information can be protected against eavesdropping or other surveillance by the use of encryption or scrambling. The terms "encryption" and "scrambling" as used herein refer generally to the process of converting "clear" or readily perceivable information, whether speech, tones or other modes of information transmission to something that is guarded or secret or not readily interpreted as meaningful information. "Encoding " is another term that has been used for the process. This is primarily a defense against intentional attempts to intercept telephone conversations or other information that is transmitted over telephone circuits. However, because of the ease of interception through the use of readily available radio receivers, otherwise illegal methods are not necessary to eavesdrop and intercept communications that are erroneously considered "safe".
Generally, the installation of voice "scrambling" or encrypting equipment at each mobile or cellular telephone that is to be involved in a sensitive communication will solve the problem. This approach, however, requires that similar equipment be installed at each participating instrument. A mobile telephone system may install a scrambler at the base station and at each of the mobile units to protect the transmissions of the system.
If a caller has sensitive telephone conversations with several different parties, it will be necessary to install similar scrambling equipment at the telephones of each of the parties. However, there are situations where it is not possible to install scrambling units at every telephone that may become party to a privileged or otherwise sensitive conversation. For example, sensitive financial information may be the subject of telephone conversations involving many different parties, as may be conversations involving trade secrets or other knowledge that should not be intercepted or overheard by persons not authorized to have access to such information.
Accordingly, there may be situations where both ends of a conversation cannot be provided with suitably matched scrambling-descrambling devices. This usually occurs in those situations where access to both instruments is neither convenient nor, in some cases, possible. In most circumstances, even if a scrambler has been installed on a cellular phone, more likely than not, the called party will not have a compatible scrambler-descrambler and the conversation would have to be without protection.
The problem has been addressed in the context of a mobile phone system and the patents to Talbot, U.S. Pat. Nos. 4,441,017, and 4,555,805, describe a communications system including a central station and a plurality of remote stations in which transmissions can either be clear (unenciphered) or secure (enciphered). The various remote stations each have a unique code which is used to encipher messages to and from that station. There is also a provision for a common code that can be used with all remote stations.
The Talbot system is generally useful with a mobile communication system that is under common control and in which communication is usually between the remote stations and the base or between individual remote stations. There is no suggestion in Talbot that the system be extended to the commercial telephone systems or that the base station be linked to a telephone switching facility.
The patent to LoPinto, U.S. Pat. No. 4,549,308, teaches an encryption key for use with cellular phones, which changes each time the carrier frequency is changed, as when a celluar subscriber leaves one cell and enters another. A key is generated from a combination which includes a non broadcast code identifier for a particular telephone. Manipulation of these and other numbers generated in the system result in an encryption key which can be used to control the creation of encryption codes which change depending upon the cell in which transmission and reception takes place.
The patent to Tobias, Jr. et al, U.S. Pat. No. 4,972,479, is directed at a system which does not require a scrambler-descrambler at each called party location. Rather, a base module with a call diverter connected to the public switched telephone network through two standard telephone lines. A bypass interface circuit includes a scrambler/descrambler which can be selectively inserted between the two lines. Under control of the calling party, the base module alternatively includes or excludes the scrambler/descrambler so that the calling party can choose as between a secure and a clear message to the base module. All voice traffic between the base module and the called party takes place over standard telephone lines and is "clear" or "descrambled".
In an alternative embodiment, Tobias, Jr. et al teach that a first base module in one geographic region can communicate with a second base module in a second geographic region using scrambler/descrambler units at each so that all long distance communication is secure. This is to insure that the microwave and radio links of the telecommunications company, which may be susceptible to eavesdropping, can be made secure by encryption or scrambling of the conversations that use the long distance services.